Search

The design of epitaxial interfaces is a pivotal way to engineer artificial structures where new electronic phases can emerge. Here, we report a systematic emergence of an interfacial superconducting state in epitaxial heterostructures of LaTiO3 and KTaO3. The superconductivity transition temperature increases with decreasing thickness of LaTiO3. Such a behavior is observed for both (110) and (111) crystal oriented structures. For thick samples, the finite resistance developing below the superconducting transition temperature increases with increasing LaTiO3 thickness. Consistent with previous reports, the (001) oriented heterointerface features a high electron mobility of 250 cm2 V−1 s−1 and shows no superconducting transition down to 40 mK. Our results imply a non-trivial impact of LaTiO3 on the superconducting state and indicate how superconducting KTaO3 interfaces can be integrated with other oxide materials.

Layered oxides have been intensively studied due to their high degree of freedom in designing various electromagnetic properties and functionalities. While Bi-based layered supercell (LSC) compounds [BinOn+δ]-[MO2] (M = Mn, Mn/Al, Mn/Fe, or Mn/Ni; n = 2, 3) are a group of prospective candidates, all of the reported compounds are insulators. Here, we report on the synthesis of two novel metallic LSC compounds [BinOn+δ]-[RhO2] (n = 2, 3) by pulsed laser deposition and subsequent annealing. With tuning the thickness of the sublattice from Bi2O2+δ to Bi3O3+δ, a dimensionality-dependent electrical transport is revealed from a conventional metallic transport in n = 2 to a localized transport in n = 3. Our successful growth will be an important step for further exploring novel layered oxide compounds.

EuTiO3 (ETO) is a unique magnetic semiconductor with a large localized magnetic moment of Eu2+ (4f 7). By the doping of high-mobility electrons in the Ti 3d conduction band, peculiar magnetotransport properties such as the unconventional anomalous Hall effect (AHE) due to Berry curvature in momentum space, as well as the Shubnikov–de Haas oscillations of spin polarized electrons, have been observed. In this study, we have examined the physical properties of high quality ETO films with La3+ (4f0) or Gd3+ (4f7) donors (ELTO or EGTO) grown on nearly lattice matched SrTiO3 substrates with a gas source molecular beam epitaxy. We find that the anti-ferromagnetic ordering of ELTO is destabilized by the vacancy of the magnetic moment on the La-site for ELTO. The maximum electron mobility for ELTO (

Carrier doping into two dimensional (2D) Mott insulators is one of the prospective strategies for exploring exotic quantum phenomena. Although ultra-thin oxide films are one such target, it is vitally important to fabricate well-defined and clean samples to extract intrinsic properties. In this study, we start from establishing the growth of clean SrVO3 films with a low residual resistivity (∼4 × 10−7 Ω cm) and a high mobility (∼103 cm2/V s). By confining them with SrTiO3 barrier layers, the Mott insulator state appears at the thickness below 3 unit cells (u.c.). By the electron doping in the form of LaxSr1−xVO3 for such two dimensional systems (2 and 3 u.c), metallic-like phases appear in a narrow x region around x = 0.17, indicating a collapse of the Mott insulator state. This study demonstrates that artificial 2D systems of clean oxides are a promising playground for exploring novel Mott physics in confined systems.

We demonstrate an electric field control of an anomalous Hall effect emerging in CaIrO3/CaMnO3 heterostructures. We fabricate both electron-type and hole-type carrier samples by tuning epitaxial strain and, then, control the carrier density in a CaIrO3 layer via electric double layer gating technique. As the Fermi energy of CaIrO3 is tuned close to the Dirac line node, anomalous Hall conductivity is enlarged in both carrier-type samples. This result reveals that the anomalous Hall effect comes from the intrinsic origin, reflecting the Dirac-like dispersion in CaIrO3. We propose that band splitting induced by the interface ferromagnetism yields several band crossing points near the Dirac line node. These points act as a source of the Berry curvature and contribute to the anomalous Hall effect.

Pyrochlore oxides A2B2O7 have been a fruitful playground for condensed matter physics because of the unique geometry in the crystal structure. Especially focusing on the A-site tetrahedral sub-lattice, in particular, pyrochlore oxides A2B2O7 (A = Sn, Pb and B = Nb, Ta), recent theoretical studies predict the emergence of the “quasi-flat band” structure as a result of the strong hybridization between filled A-ns and O-2p orbitals. In this work, we have established the growth conditions of Sn2Nb2O7, Sn2Ta2O7, Pb2Nb2O7, and Pb2Ta2O7 films by pulsed laser deposition on Y-stabilized ZrO2 (111) substrates to elucidate their optical properties. Absorption-edge energies, both for direct and indirect bandgaps, increase in the order of Sn2Nb2O7, Sn2Ta2O7, Pb2Nb2O7, and Pb2Ta2O7. This tendency can be well explained by considering the energy level of the constituent elements. A comparison of the difference between direct and indirect bandgaps reveals that Pb2B2O7 tends to have a less dispersive valence band than Sn2B2O7. Our findings are consistent with the theoretical predictions and are suggestive of the common existence of the hybridized states in this class of compounds.

Sn2Nb2O7, a pyrochlore oxide, is expected to have a peculiar flat band structure that can be a source of intriguing physical properties. We have prepared Sn2Nb2O7 thin films by pulsed laser deposition on Y-stabilized ZrO2(111) substrates by finding a growth window at low growth temperature (∼450 °C) and higher laser fluence, possibly due to the rather high volatility of stannous oxide. By Ti-doping, the lattice constant shrinks due to the smaller Ti4+ ion compared with Nb5+. Optical measurements capture an absorption edge at ∼2.6 eV originating from the transition from the Sn-5s and O-2p hybridized valence band to the Nb-4d conduction band, which is not observed in LiNbO3. Opposed to the expectation from the Burstein–Moss shift, Ti-doping results in reduction of the bandgap, indicating that the doping results in the formation of in-gap states and does not contribute to injecting mobile carriers.

While the family of layered pnictides ABX2 (A—rare or alkaline earth metals, B—transition metals, and X—Sb/Bi) can host Dirac dispersions based on Sb/Bi square nets, nearly half of them have not yet been synthesized for possible combinations of the A and B cations. Here, we report the fabrication of EuCdSb2 with the largest B-site ionic radius, which is stabilized for the first time in thin film form by molecular beam deposition. EuCdSb2 crystallizes in an orthorhombic Pnma structure and exhibits antiferromagnetic ordering of the Eu magnetic moments at TN = 15 K. Our successful growth will be an important step for further exploring novel Dirac materials using film techniques.

We have investigated the charge transport in thin films of correlated Dirac semimetal of perovskite CaIrO3 by measurements of resistivity and optical spectra. The semimetallic transport of either electron-type or hole-type carrier is observed in the strain-relaxed thin films. By controlling the strain relaxation via thermal annealing, the carrier density decreases in both n-type and p-type samples, while enhancing the carrier mobility up to 160 cm2 V−1 s−1 at an electron density of 2.5 × 1018 cm−3 at 2 K. We propose that the energy of Dirac line node, which locates in proximity to the Fermi level, varies with the lattice distortion or strain-sensitive defect character, causing the sign change in the charge carrier as well as the mobility enhancement.

Rapid progress of quantum transport study in topological Dirac semimetal, including observations of quantum Hall effect in two-dimensional (2D) Cd3As2 samples, has uncovered even more interesting quantum transport properties in high-quality and three-dimensional (3D) samples. However, such 3D Cd3As2 films with low carrier density and high electron mobility have been hardly obtained. Here, we report the growth and characterization of 3D thick Cd3As2 films adopting molecular beam epitaxy. The highest electron mobility (μ = 3 × 104 cm2/Vs) among the reported film samples has been achieved at a low carrier density (n = 5 × 1016 cm−3). In the magnetotransport measurement, Hall plateau-like structures are commonly observed despite the 3D thick films (t = 120 nm). On the other hand, the field angle dependence of the plateau-like structures and corresponding Shubunikov-de Haas oscillations rather shows a 3D feature, suggesting the appearance of an unconventional magnetic orbit, also distinct from the one described by the semiclassical Weyl-orbit equation.

Ferromagnetic van der Waals (vdW) insulators possess robust magnetic order even in a few layers of two-dimensional sheets. The heterostructures of such vdW materials prepared by molecular-beam epitaxy (MBE) are one of the ideal platforms for exploration of novel electronic/spintronic functionalities based on their ferromagnetism via an abrupt hetero-interface. Here we report successful MBE-growth of a vdW magnetic insulator Cr2Ge2Te6 thin film on a topological insulator (Bi,Sb)2Te3. Metal to insulator transition is observed in Cr–Ge–Te alloy films with increasing Ge content as tuned by the Ge and Cr flux ratio, corresponding to the structural phase change from Cr2Te3 to Cr2Ge2Te6. In the nearly stoichiometric Cr2Ge2Te6 films, a large remanent magnetization with perpendicular magnetic anisotropy appears in contrast to the bulk crystals with no discernible hysteresis. The perpendicular remanence with high Curie temperature of about 80 K remains in the thinnest 6-nm film prepared in this study. Designing of magnetic vdW heterostructures based on the Cr2Ge2Te6 thin films offers great opportunities for exploring unusual physical phenomena via proximity effect at the vdW hetero-interface.

We report the growth of superconducting Sr2RuO4 films by oxide molecular beam epitaxy (MBE). Careful tuning of the Ru flux with an electron beam evaporator enables us to optimize growth conditions including the Ru/Sr flux ratio and also to investigate stoichiometry effects on the structural and transport properties. The highest onset transition temperature of about 1.1 K is observed for films grown in a slightly Ru-rich flux condition in order to suppress Ru deficiency. The realization of superconducting Sr2RuO4 films via oxide MBE opens up a new route to study the unconventional superconductivity of this material.